linux-kernel - Re: [PATCH 4/5] KVM: MMU: store generation-number into mmio spte

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Message-ID: <20130320184329.GA3888@amt.cnet>
Date:	Wed, 20 Mar 2013 15:43:29 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 4/5] KVM: MMU: store generation-number into mmio spte

On Mon, Mar 18, 2013 at 08:42:09PM +0800, Xiao Guangrong wrote:
> On 03/18/2013 07:19 PM, Paolo Bonzini wrote:
> > Il 15/03/2013 16:29, Xiao Guangrong ha scritto:
> >> +/*
> >> + * spte bits of bit 3 ~ bit 11 are used as low 9 bits of
> >> + * generation, the bits of bits 52 ~ bit 61 are used as
> >> + * high 12 bits of generation.
> >> + */
> > 
> > High 10 bits.
> 
> Yes, i forgot to update the comments! Thank you, Paolo!
> 
> > 
> > How often does the generation number change?  Especially with Takuya's
> > patches, using just 10 bits for the generation might be enough and
> > simplifies the code.
> 
> I guess it's only updated when guest is initializing and doing hotplug.
> In my origin thought, if the number is big enough that it could not
> round on all most case of hotplug, the things introduced by Takuya is
> not necessary. But, if you guys want to keep the things, 10 bits should
> be enough. :)

Since an overflow can happen in normal scenarios (cirrus emulation can
delete/add memory slot on legacy mode, with syslinux, for example), it
would be good to handle the overflow scenario. It also means bits for
generation number can be reduced.

Can walk the GPA range being added/moved instead of walking
active_mmu_pages (should be much faster since usually its a small slot
being added/deleted).

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